In general, gasoline and other liquid hydrocarbon fuels boil in the range of about 100.degree. F. to 650.degree. F. However, the crude oils from which these fuels are made contain mixtures of hydrocarbons which boil over wider temperature ranges, the boiling point of each hydrocarbon depending upon its molecular weight. As an alternative to discarding or otherwise using the higher boiling hydrocarbons, the petroleum refining industry has developed a variety of processes for breaking or cracking the large molecules of high molecular weight into smaller molecules which boil within the above boiling range for hydrocarbon fuels. The cracking process which is most widely used for this purpose at the present time is known as fluid catalytic cracking (FCC) and may employ a fluidized bed reactor with backmixing and/or a riser reactor with progressive flow. In a typical FCC process, feedstock oil is mixed with particulate catalyst at an elevated temperature in the lower portion of an elongated reaction vessel called a "riser". Contact of the hot catalyst with the oil rapidly generates large volumes of gases which propel the stream of feed and catalyst as a suspension through the reaction zone at high velocity, giving relatively short contact times.
The initial propelling gases are comprised of vaporized oil, the major portion of which boils below 1,025.degree. F. and is immediately vaporized by contact with the hot catalyst which enters the riser at a higher temperature. As the suspension travels up the riser, a large fraction of the feedstock hydrocarbons is converted to lower boiling hydrocarbons by catalytic cracking and these cracked products form part of the propelling gases. The velocity of the suspension is sometimes increased further by introducing diluent materials into the riser either along with the feed or separately. The conversion reaction initiated in the lower portion of the riser continues until the catalyst and gases are separated, which may take place as the suspension leaves the riser reaction zone or in an upper, larger diameter vessel for collecting the catalyst. Upon being separated from the catalyst, the gases are usually referred to as "product vapors".
Crude oil in its natural state contains a variety of materials which, unless removed prior to the cracking reaction, tend to have troublesome effects on FCC processes. These include coke precursors, such as asphaltenes, polynuclear aromatics and high boiling nitrogen containing molecules; and metals, such as sodium and small amounts of other alkali or alkaline earth metals, nickel, vanadium, iron and copper, which are detrimental to the conversion process and/or to the catalyst.
During the cracking operation, coke precursors either tend to deposit as solid aromatic structures having some residual hydrogen or are high boiling and do not vaporize but lay down on the catalyst as a liquid. These coke deposits block the catalytically active acid sites of the catalyst and thereby reduce its conversion activity. While it is believed that both the solid and liquid components of coke may cover and thereby block acidic sites, the liquid components may also fill pores of the matrix and thereby retard diffusion of lower boiling components to the zeolite. Although the carbonaceous material formed by the conversion process is referred to as coke, it may have hydrogen to carbon ratios of 1.0 or greater and may contain in addition to hydrogen various amounts of other element depending upon the composition of the feed. The coke formed is deposited on the catalyst particles and thereby reduces the conversion activity of the catalyst. In order to restore conversion activity, the contaminated catalyst is regenerated by burning off the coke by contacting the catalyst particles at relatively high temperatures with an oxidizing gas such as air. The regenerated catalyst may then be returned to the reaction zone for additional passes or conversion cycles in contact with fresh feed.
In general, the coke-forming tendency or coke precursor content of a feedstock oil can be ascertained by determining the weight percent of residue remaining upon pyrolyzing a sample of the feed. Two tests presently recognized by the industry are the Conradson carbon residue test described in ASTM D189-76 and the Ramsbottom carbon test described in ASTM D524-76. In conventional FCC practice, Conradson carbon residues of about 0.05 to 1.0 are regarded as indicative of relatively contaminate free gas oil feeds.
Unless removed by careful desalting of the crude oil, the sodium, and other alkali or alkaline earth metals can diffuse to the active, i.e., acidic, sites of the catalyst and poison or kill their catalytic activity. Vanadium, and to a lesser extent nickel and other metals, may also migrate to and poison acidic sites. There metals will therefore be referred to collectively as poison metals. Nickel, vanadium, copper and iron are also known as "heavy metals" and catalyze undesirable dehydrogenation reactions which increase the amount of coke deposited on the catalyst and the amounts of hydrogen and normally gaseous hydrocarbons to be handled by process equipment. During the cracking process, the heavy metals transfer almost quantitively from the feedstock oil to the catalyst particles and tend to deposit on interior and exterior surfaces of the particles where they can block and/or retard diffusion to active sites.
Since the various heavy metals are not of equal poisoning activity relative to catalytic acid sites, it is convenient to express the poisoning activity of an oil containing one or more of these metals in terms of the amount of a single metal which is estimated to have equivalent poisoning activity. Thus, the heavy metals content may be expressed by the following formula in which the content of each metal present is expressed in parts per million by weight based on the weight of the oil: Nickel Equivalents=Ni+V/4.8+Fe/7.1+Cu/1.23. In conventional FCC practice, crude oils are carefully fractionated to provide a gas oil with a relatively low, level of heavy metal contaminants, namely, 0.25 ppm Nickel Equivalents or less.
The above formula can also be used as a measure of the heavy metals accumulated on the cracking catalyst itself, the quantity of metal used in the formula being based on the weight of moisture-free catalyst. In FCC practice, equilibrium catalyst is removed and fresh, contaminant-free catalyst is added at a rate sufficiently high to control the heavy metal content of the catalyst at relatively low levels, namely, 1,000 ppm Nickel Equivalents or less.
Some crude oils contain from about 10% to about 30% by volume of heavy hydrocarbons which will not boil below about 1,025.degree. F. at atmospheric pressure. Atmospheric bottoms and vacuum bottoms may contain even higher percentages of this highest boiling fraction. The coke precursor and poison metal components of the crude are for the most part concentrated in this fraction. Accordingly, many of the problems presented by these components have been avoided by sacrificing the yield of liquid fuel fractions which is potentially available from cracking the highest boiling fraction. More particularly, in conventional FCC practice, the crude oil has been vacuum fractionated to provide a FCC feedstock boiling between about 650.degree. F. and about 1,000.degree. F., this fraction being referred to as a vacuum gas oil and being relatively free of coke precursors and poison metals. Vacuum gas oil is generally prepared from crude by distilling off the fraction boiling below about 650.degree. F. at atmospheric pressure and then separating by vacuum distillation one or more fractions boiling between about 650.degree. F. and about 1,025.degree. F. from the heaviest fraction boiling above 1,025.degree. F. The heaviest fraction is normally not used as a source of catalytic conversion feedstock, but instead is employed for other purposes, such as the production of asphalt, which represents a waste of the potential value of this portion of the crude oil as a source of liquid fuels.
Due to the continually increasing demand for gasolines, relative to heavier liquid fuels, coupled with shrinking supplies of normally used gas oil cracking stocks, more attention has recently been given to the catalytic cracking of heavier chargestocks, such as residuals from which the highest boiling fraction has not been separated. In addition, consideration has been given to blending the heaviest or "resid" fraction with various lower boiling fractions in order to increase overall conversion of crude oil to liquid fuels. In view of the high potential value of the heaviest fraction of crude oils, a number of methods have been proposed in the past to overcome the problems associated with the cracking of feedstocks contaminated with metals and coke precursors and thereby increase the overall yield of gasoline and other hydrocarbon fuels from a given quantity of crude oil. Suggestions have been made to pretreat the contaminated feed to reduce the metals content to below about 4 ppm nickel equivalents and the Conradson carbon residue to below about 1. Various demetalization techniques have also been suggested for removing the metal contaminants once they have been deposited on the catalyst. Most of these prior art techniques, however, require expensive additional equipment and materials and cannot be justified from an economic standpoint.
Attention has also been given to developing improved catalysts for cracking more contaminated feeds. However, many problems have been encountered in the use of prior art catalysts for cracking feeds containing a resid fraction.
A catalyst comprised of crystalline zeolite particles embedded within a larger matrix particle has numerous passages leading from the outer peripheral surface of the matrix particle to the smaller zeolite particles supported within the matrix. In this specification, these matrix passages are referred to as "feeder pores". Feeder pores in effect provide access passageways from the surface of each catalyst particle to those zeolite particles at locations internal to the matrix. There also may be a small but finite number of zeolite particles exposed at the surface of the matrix.
Generally, the pores of zeolitic sieves fall within the range of 4 to 13 .ANG.. Accordingly, any pores larger than 13 .ANG. are usually in the matrix. In prior art catalysts of this type, the average diameter of feeder pores in fresh catalyst usually falls within the range of about 30 .ANG. to about 400 .ANG.. Alumina-silica matrices generally have pores in this range, although a relatively small proportion may be larger. However, after an extended period of use, the effective average pore diameter of these prior art catalysts may be decreased significantly because of coke and metal accumulations. These prior art catalysts have proven inefficient for cracking resid containing feedstocks for a number of reasons, including both a low zeolite utilization factor and undesirable reaction diffusion limitations. The low zeolite utilization effect is a consequence primarily of the deposition of both coke and heavy metals in and/or across the mouths of the working pores of the zeolite. These components in effect block off zeolitic pore volume containing unused or incompletely used acidic sites.
"Zeolitic pore volume" refers to the free volume of the micropores in the zeolite component rather than the matrix. The term "pore volume" as applied to the catalyst composition as a whole refers to the free volume in the matrix and zeolite combined which is provided by both macropores (pores having a minimum diameter above 30 .ANG.) and micropores (pores having a minimum diameter of 30 .ANG. or less). The pore volume fraction for pores greater than 30 .ANG. in diameter may be determined by mercury porosimetry methods, such as the method of U.S. Pat. No. 3,853,789.
The pore volume fraction in the 0 to 30 .ANG. range may be determined by the BET nitrogen adsorption method described by Brunauer, Emmett, and Teller in the Journal of the American Chemical Society, 60, 309 (1938). The pore volume of fresh hydrocarbon conversion catalyst may vary widely depending upon the size of the pores in the matrix and, where used, the relative amount of catalytic promoter, such as zeolite, and the size of its pores.
Diffusion limitations may result from a number of different mechanisms. One such limitation is a consequence of high molecular weight molecules in the feed and the absence of a sufficient number of feeder pores of the size range required for transporting these large molecules to the acidic sites of the catalyst, some of which may be in the matrix but the majority of which are in the zeolite. Another diffusion limitation in the processing of reduced crude or other resid containing feeds is due to what may be called "pore plugging". Pore plugging is caused by the absorption of unvaporized hydrocarbons in the catalyst pores so that they are impractical to remove by stripping operations prior to regeneration. The trapping of heavy hydrocarbons which cannot be removed by conventional stripping operations can lead to excessive coke and regeneration temperatures and increased air consumption. Pore plugging and the deposition of coke and/or heavy metals within or over the pores also leads to decreased diffusion of reactants to and products from acidic sites. Slow diffusion rates may result in thermal cracking predominating over catalytic cracking, which in turn causes loss of selectivity. Thus, catalysts possessing relatively small or restricted feeder pores will show relatively poor cracking characteristics when cracking resid containing feeds, including low conversion, poor selectivity, increased air consumption during regeneration, and higher regeneration temperatures. Hot spots also occur more readily during regeneration and cause catalyst deactivation through sintering of the matrix and loss of zeolite crystalline structure and acidity. Furthermore, low catalyst utilization factors and diffusion limitations both require high catalyst to oil ratios which necessitate relatively low oil feed rates.
In order to provide economic levels of conversion activity and more importantly the selectivity required in processing the very refractory hydrocarbons found in resid fractions, it is desirable to run the riser at a relatively high temperature. In addition, large amounts of coke accumulate on the catalyst. The primary problem with this increased coke make is that the reactions in the regenerator which convert coke to carbon monoxide and carbon dioxide are highly exothermic. Since the regeneration reactions are exothermic, the regeneration step is normally carried out at a temperature much above the cracking temperature in the riser. This makes it necessary to run the regenerator at maximum temperatures in order to burn the coke off the catalyst to the relatively low levels required for restoration of its activity. To achieve a heat balance in cracking resid, it is therefore necessary to operate the regenerator at very severe hydrothermal conditions, which can cause rapid degradation of many prior art catalysts.
At high regenerator temperatures, excess heat and localized hot spots may develop within catalyst particles, especially in places where pore plugging has occurred or excessive coke deposits have accumulated. These localized hot spots result in sintering and collapse of the matrix pore structure, thus rendering a large portion of the acidic sites in the matrix unavailable for further reactant contact. Where a catalytic promoter is used, the promoter will necessarily be entrapped within the collapsed pores of the matrix and blocked off from further reactive contact. Coke from resid molecules can also cover and block portal surface areas of both the matrix and the zeolite.
The crystalline structure of zeolites is susceptible to degradation by high regenerator temperatures per se. Zeolites are crystalline alumino-silicates made up of tetra-coordinated aluminum atoms associated through oxygen atoms with silicon atoms in an ordered crystalline structure. Localized hot spots in or near the zeolite particles can cause destruction of the aluminosilicate crystalline structure, at least to the extent of destroying portal area of the zeolite, with a resulting loss of its catalytic action. Furthermore, both sodium and vanadium contaminates accelerate sintering and collapse of pore structures in both the matrix and zeolite components. Such degradation permanently deactivates the catalyst so that it must be removed from the system, resulting in high make-up rates that may prove uneconomical because of the high cost of zeolite in the catalyst. There is a need therefore for a heat resistant zeolite catalyst suitable for use in cracking resid containing oil feeds with improved overall utilization of acidic sites and a minimum of diffusion limitations.
While it has been recognized in the past that the physical structure of catalyst particles plays an important role in their effectiveness, the extent to which such structure is important generally has been obscured by the lack of analytical techniques for isolating the complex mechanisms involved in catalytic cracking. In this connection, some attention has been given in the prior art to increasing the pore size of catalyst matrices. Thus, it has been suggested that extremely large pores, such as those with diameters above 1,000 .ANG., might be introduced into a catalyst by incorporating a removable material and subsequently removing that material during catalyst preparation. See, for example, the catalysts described in U.S. Pat. No. 2,890,162 to Anderson, et al., and U.S. Pat. No. 3,944,482 to Mitchell, et al. However, the removable materials suggested for this purpose have not been easily controllable and have resulted in poorly defined pore structures and a wide variety of pore sizes, relatively few, if any, of the pore diameters being in the actual size range needed for resid cracking.